Improved high-strength bainitic hot-rolled strip for complexly shaped components

Picture of a traverse control arm — Fig. 1: Traverse control arm

The automotive industry uses bainitic hot rolled strip such as SZBS800 with a tensile strength of 800 MPa and thicknesses from 2 - 4 mm to manufacture chassis parts, such as transverse control arms (see Fig. 1).
A good steel product is characterized by two considerations: On the one hand, the defined characteristics of the component to be manufactured and on the other hand, good processability in the user's manufacturing process.
One test that establishes the material characteristics is the tensile test, which supplies the strength and strain values (tensile strength, yield strength, elongation at break) under a single-axis tensile load.

One of the most common methods used at this time to determine the processability of chassis elements is the hole expansion test in accordance with ISO 16630, in which a hole is punched into the material and then expanded by means of a tool. The determined characteristic indicates the point in the expansion at which a crack develops on the punched sheet edge. The hole expansion ratio consequently represents potential damage during a component's manufacturing that is preceded by a significant plastic load on a shear cut edge.
While the damage develops in the sample interior in the tensile test, the hole expansion test results in the creation of a crack in the edge area that has already been strained by the cutting. By way of illustration, the test samples used in the tests are shown in Fig. 2.

Pictures of the test samples from the tensile test and the hole expansion test. — Fig. 2: Test samples from the tensile test (left) and the hole expansion test (right).

Material development's job is to set up material structures that allow both high elongation at break values as well as high hole expansion ratios at the required tensile strength of 800 MPa. A special challenge here is to increase the comparatively low hole expansion ratio at a high strength by homogenizing the structure while simultaneously maintaining the high elongation at break level. The correlations of the to some extent contrary characteristics strength, elongation at break and hole expansion are illustrated in Fig. 3.

Schematic figure of the correlation between hole expansion value and tensile strength, and correlation between elongation at break and hole expansion — Fig. 3: General correlation between hole expansion value (ordinate) and tensile strength (abscissa), left, and correlation between elongation at break and hole expansion in a comparison of dual and complex phase steels with the same strength.

The optimal bainitic structure is achieved by coordinating the analysis concept and process parameters. This requires that the structure be characterized at different size scales.
The hole expansion level can be significantly increased by systematically optimizing the bainitic structure. Implementing the corresponding measures leads to an increase in the hole expansion ratio of 27% points, as shown in Fig. 4. It was consequently possible to improve the Salzgitter Flachstahl SZBS800 even further with regard to the hole expansion ratio. The SZBS80xpand grade provides the customer with optimized processability and outstanding strength and strain values.
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